There is considerable variation in cooking times among microwave ovens even when considering only a particular model of a given manufacturer. The dominant factor for this variation is differences in the output powers of the magnetrons of the respective ovens; these differences result primarily from differences in the powers provided by their respective power supplies. The power delivered to the magnetron in the nearly universal power supply design depends on the effective turns ratio of the plate transformer and the effective value of the storage capacitor. While it would be possible to measure and pair these components to produce a standard plate current, the process for doing such would be very expensive. Further, the power output of a given power supply would vary substantially as a function of ac line voltage which typically may vary by as much as 30% in domestic applications. It would be possible to overcome the output variance as a function of ac line voltage, but the precision power supply required would be prohibitively expensive. In short, the relatively inexpensive power supply design used in most domestic microwave ovens results in ovens of the same model producing various output powers even when operated with a regulated ac line voltage. For example, magnetron output powers supplied by the power supplies with a regulated ac line voltage for a particular model may vary from 600-750 watts with an average of approximately 670 watts. Further, an individual oven will exhibit a significant swing in output power as a result of changes in the ac line voltage.
Variation in microwave cooking times described heretofore has created problems for the microwave cooking industry. For example, manufacturers of prepackaged foods are unable to provide accurate cooking directions and may lose customers if the results are not satisfactory. Also, the user precisely following a cook book recipe and the cooking time provided therein will be dissatisfied if the food is overdone or underdone. Furthermore, with state of the art cook-by-weight ovens, the microprocessor algorithm for calculating cooking times preferably includes a term derived from the predicted output power of the magnetron.
The cooking time variance with microwave ovens is much more critical than with conventional gas or electric ovens where the cooking times are substantially a function only of the accuracy of the thermostat; the times do not vary additionally as a function of the ac line voltage. Further, in most conventional ovens, inconsistencies between the oven temperature and the dial set temperature can be corrected by a simple adjustment to the dial. Also, users have developed an understanding for how to compensate conventional cooking times when the oven is consistently not hot enough. However, the same understanding is generally not present with users who may be new to microwave cooking; this is especially true in view of the cooking time variation with a given microwave oven as a function of the ac line voltage.
From the foregoing, it is apparent that it is desirable to provide microwave ovens having constant uniform output powers to establish standard predictable cooking times. One prior art approach to the general problem of non-uniform cooking times is to monitor the ac line voltage and recalculate the preset cooking time as a function thereof. Although this approach may provide some improvement for the cooking time variation as a function of a varying ac line voltage, it provides no correction for cooking time variation caused by differences in components of the power supplies of respective microwave ovens.